chg: nil: Move application of dns64 to a separate function

The code in `query_dns64()` that applies the dns64 prefixes to an A rdataset has been moved into the `dns_dns64` module, and `dns_dns64_destroy()` now unlinks the dns64 object from its containing list.

With these changes, we no longer need the list-manipulation API calls, `dns_dns64_next()` and `dns_dns64_unlink()`.

Merge branch 'each-simplify-dns64' into 'main'

See merge request isc-projects/bind9!10324